Study of the Nature of High-Silica HY Acid Sites in Dimethoxymethane

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Study of the Nature of High-Silica H-Y Acid Sites in Dimethoxymethane Carbonylation by NH3 Poisoning Zhiqiang Xie, Congbiao Chen, Bo Hou, Dekui Sun, Heqin Guo, Jungang Wang, Debao Li, and Litao Jia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00421 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Study of the Nature of High-Silica H-Y Acid Sites in Dimethoxymethane Carbonylation by NH3 Poisoning Zhiqiang Xiea,b, Congbiao Chena, Bo Houa, Dekui Suna, Heqin Guoa, Jungang Wanga, Debao Lia, Litao Jiaa,* a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, Taiyuan 030001, P.R. China. b

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

*Corresponding author. Email address: [email protected].

1

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Abstract The nature of high-silica H-Y acid sites in dimethoxymethane carbonylation is investigated by NH3 poisoning with a combination of reaction results, diffuse reflection

infrared

Fourier

transform

spectra

and

NH3

temperature

programmed desorption. After NH3 poisoning of H-Y, ammonia desorption of zeolites NH4-Y was conducted at different temperatures in order to obtain catalysts HNH4-Y that gradually exposed different acid sites. The zeolites thus produced

catalyzed

dimethoxymethane

carbonylation

with

a

linear

dependence of the conversion on the ammonia desorption temperature before the plateau stage. It was observed that strong Brønsted acids in zeolite Y catalyzed the reaction more selectively toward the main product methyl methoxyacetate and weak Brønsted acids catalyzed the reaction more selectively toward the byproduct methyl formate. Furthermore, the turnover frequency of Brønsted acids varied significantly among the Brønsted acids and decreased with acid strength. This finding suggested that only 10.1%-18.3% of the total Al in zeolite Y with Si/Al ratios of 15, 30 and 40 contributed to the total framework of Brønsted acids.

2


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1. Introduction Ethylene glycol (EG) is widely used as antifreeze and a raw material in important industrial processes, such as producing polyester fibers and cosmetics. EG is produced largely by the hydration of ethylene oxidation in the petrochemical industry. Given concerns regarding decreasing petroleum resources, other methods to obtain EG are required.1 EG production with syngas (CO and H2) as a raw material has drawn increasing attention as a possible alternative. Syngas can be supplied in abundance from natural gas, coal gasification or biomass treatment. One of the extensively studied EG production methods from syngas is hydrogenation of dimethyl oxalate that can be obtained through coupling CO and methyl nitrate.2-4 The route requires NO recycling process and prevention of possible nitric acid corrosion. Other promising EG production routes based on syngas include

carbonylation

of

formaldehyde

or

its

dimethyl

acetal,

dimethoxymethane (DMM), as a key step. Formaldehyde and DMM can be synthesized by the partial oxidation of methanol. The primary products of formaldehyde carbonylation and DMM carbonylation are glycolic acid and methyl methoxyacetate (MMAc), both of which are precursors to EG. Formaldehyde carbonylation is mostly performed with a large amount of liquid organic solvents in a batch reactor and requires high CO pressure.5-10 In contrast, DMM carbonylation can be performed under low CO pressure in the vapor phase in a fixed bed reactor, which is possible for large-scale industrial production. In addition, DMM carbonylation is an efficient and environmentally friendly method to produce an ethylene glycol precursor. Alexis T. Bell and coworkers first reported the DMM carbonylation reaction with acid zeolites as catalysts.11-14 In addition to the main product of MMAc, the side-reaction is DMM disproportion, which produces dimethyl ether (DME) and methyl formate (MF). When high-silica H-Y was used as a catalyst, an MMAc selectivity of 79% 3



and a DMM conversion of 20% were reached under mild conditions. Theoretical studies and infrared spectroscopy have demonstrated that the DMM carbonylation reaction involves carbocationic transition states stabilized by oxygen atoms of the zeolite framework. It has also been reported that the zeolite framework type and Si/Al ratio significantly influence DMM carbonylation.13 In addition to acid zeolites, Zhongmin Liu and coworkers developed a Nafion-H catalyst and Nafion-silica composite catalyst for DMM carbonylation with MMAc selectivity as high as 90%.15,16 The catalysts’ excellent catalytic results are due to their high acid strength and large pores. Present literature regarding DMM carbonylation have focused mainly on the pore structure and acid strength of the catalyst. Studies regarding other factors, such as the role of different kinds of acid sites in DMM carbonylation, are needed. When H-Y zeolite was applied in DMM carbonylation, a high temperature pretreatment was typically needed both to desorb water and to transform the ammonium form of zeolite into the hydrogen form before the reaction. After pretreatment, all Brønsted acids were accessible and contributed to the reaction as active sites at the same time. Because of the heterogeneity of the sites, the reaction results were actually caused by different Brønsted acids, without distinguishing the influence of different active sites. Another problem has been regarding the determination of the framework Brønsted acid amount when the reaction rate of DMM carbonylation was required. Since different framework Al structures, such as framework Lewis acids,17,18 have been demonstrated to exist on zeolites, the framework Brønsted acid amount cannot simply be calculated as the framework Al amount. In the present work, to further explore the Brønsted acid nature of high-silica zeolite Y and its effects on DMM carbonylation, a convenient NH3 poisoning method was used in this work. The ammonia-covered zeolite NH4-Y 4


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was heated at different temperatures to desorb NH3 and was later cooled to be tested in situ in DMM carbonylation. It was possible to highlight the effects of different Brønsted acids exposed at different temperatures on the reaction and to calculate the amount of active Brønsted acids. Diffuse reflection infrared Fourier transform spectra (DRIFTS) and NH3 temperature programmed desorption (NH3-TPD) were also applied to investigate the nature of the Brønsted acids.

2. Experimental Section 2.1. Catalysts pretreatment Zeolites H-Y with nominal Si/Al ratios of 15, 30 and 40 were purchased from Alfa Aesar, which are denoted as Y-15, Y-30 and Y-40, respectively. Part of the zeolite Y-15 was calcined statically in air at 500℃ for 4 hours before use and is denoted as Y-15-C. To remove the extra-framework aluminum species, part of Y-15-C was treated with 0.06mol/L ethylenediaminetetraacetic acid disodium salt dihydrate (Na2EDTA, 98%, Aladdin, China), 30mL/g zeolites, at 90℃ for 24h while stirred. After thorough washing by deionized water, the zeolites were exchanged with 1 mol/L NH4NO3 (AR, Kemiou, China), dried at 120℃ and calcined in air at 500℃ for 4 hours. This process was repeated once. The zeolites that were prepared in this way were denoted as Y-15-CE.

2.2. DMM carbonylation test DMM (98%, Aladdin, China) carbonylation tests were performed on a stainless steel fixed-bed reactor with inner diameter 6 mm. DMM was bubbled into the reactor at a rate of 0.09 mol/h using CO (99.9%, Yihong, China) as a carrier gas. The CO flow rate and reaction pressure were controlled by a mass flow controller and pressure regulators, respectively. In a typical experiment, 5



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50 mg zeolite particles that were 0.18-0.25 mm were pretreated at 500℃ under Ar for four hours before being treated by 1% NH3/Ar at 50℃ for one hour. The zeolite was heated under Ar to temperature x (in which x > 120℃), and temperature x was kept constant for 30 minutes to desorb NH3. The zeolite that was obtained in this way was used in situ to catalyze DMM carbonylation and the reaction proceeded under fixed conditions: reaction temperature of 120℃, reaction pressure of 1.0 MPa and CO flow rate of 80mL/min (standard temperature and pressure). When temperature x changed, zeolites with different amounts of Brønsted acids were obtained and applied to the reaction. The zeolite used in DMM carbonylation was changed with new catalyst for each temperature x test. Reaction products were analyzed by gas chromatography (GC-950, Haixin, China) with an HP-INNOWAX capillary column and flame ionization detector. Three carbon atoms of MMAc originate from DMM;13 therefore, the selectivity of MMAc was calculated as SMMAc =

ଷ௡ಾಾಲ೎ ଶ௡ವಾಶ ାଶ௡ಾಷ ାଷ௡ಾಾಲ೎

, where nx

was the amount of product x. Product analysis was performed within first two hours (more time was required for catalysts with high ammonium coverage) when a relatively steady state was achieved. The material balance ascribed to DMM was within 100 ± 5%.

2.3. Catalyst characterization NH3-TPD was performed on a quartz tube (6mm inner diameter) with a mass spectrometry detector. In accordance with the DMM carbonylation test, NH3-TPD with one constant temperature stage was used to determine the amount of NH3 that was desorbed above the threshold temperature (230℃, 200℃ and 190℃ for Y-15, Y-30 and Y-40, respectively). After NH3 desorption 6


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was performed at the threshold temperature, DMM conversion on the zeolite was approximately 4%. Typically, 0.1 g zeolite was first calcined at 500℃ for one hour under argon (99.9%, Yihong, China) flow. At 50℃, the zeolite was treated with 1% NH3 (99.9%, Zhongrui, China)/Ar for one hour, and was then purged with Ar for one hour. TPD began with a heating rate of 10℃/min to the threshold temperature under 80 mL/min Ar. After the threshold temperature was maintained at a constant for 30 minutes, TPD continued with a heating rate of 10℃/min to approximately 580℃. Conventional NH3-TPD without a constant temperature stage was also performed to ensure that the area difference was within 5%. The amount of desorbed NH3 was quantified by both the titration method and ammonium carbonate decomposition. DRIFTS were recorded by a Nicolet iS10 with mercury cadmium telluride detector cooled by liquid nitrogen. Zeolite was placed in the in situ cell crucible and treated under Ar at 500℃ for 4 hours. When the zeolite was cooled to 100℃, DRIFTS were recorded by averaging 64 scans at a resolution of 4 cm-1 in the region of 4000 to 400 cm-1. The spectrum was used as background. Then, the zeolite was treated with 1% NH3/Ar for one hour and purged with Ar for one hour at 100℃. Then the zeolite was heated under Ar at a rate of 10℃ /min to several increasing temperatures that were chosen according to the DMM conversion. In accordance with the DMM carbonylation test, each of these temperatures was kept constant for 30 minutes to desorb NH3 and when the zeolite was cooled to 100℃ again, DRIFTS were recorded with the abovementioned background. The zeolite was then heated to the next temperatures and DRIFTS were recorded at 100 ℃ after each constant temperature desorption. Except for the broad stretching vibration of 7



ammonium, only spectra in the hydroxyl group region were clearly recognized and reported in present work. All DRIFTS reported were repeated once and no noticeable differences were observed. An NH3 desorption time of two hours at each constant temperature stage, instead of 30 minutes in the present work, was also tested and the resulting spectra were the same. Pyridine Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet 6700 with a vacuum system of 10-3 Pa. A self-supporting zeolite wafer (10 mg/cm-1) was degassed at 450℃ under a vacuum for 2 hours. After absorbing Pyridine and being degassed under a vacuum, the sample was heated to 150 ℃ to desorb Pyridine and FTIR were recorded at room temperature.

27

Al MAS NMR experiments were performed with a Bruker

Avance III 600 MHz Wide Bore spectrometer (14.2T) using Al(NO3)3 as reference. Si and Al contents were determined by inductively coupled plasma atomic emission spectroscopy. BET specific areas were calculated from N2 adsorption-desorption isotherms on Micromeritics ASAP 2460.

3. Results 3.1. DMM carbonylation results Fig. 1 shows the influence of NH4-Y ammonia desorption temperature (ADT) on DMM conversion and product selectivity under fixed reaction conditions. As shown in Fig. 1a, the reaction did not occur until NH3 was desorbed from the zeolites above the threshold temperatures of 230℃, 200℃ and 190℃, at which DMM conversion was 3.6%, 3.7% and 3.8% for Y-15, Y-30 and Y-40, respectively. Since the reaction temperature (120℃) was much lower than ADT in Fig. 1, NH3 residue that was absorbed on the zeolites was expected to be sufficiently steady not to continue to desorb during

8


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reaction process. After NH3 was desorbed to some extent (i.e. 400℃, 320℃ and 260℃), the zeolites reached conversion plateaus with the highest DMM conversions being 77%, 78% and 74% for Y-15, Y-30 and Y-40, respectively. These plateau conversion values were the same as zeolites without NH3 pretreatment, suggesting that all the active sites could be recovered to full activities. Before reaching the plateau, DMM conversion increased almost linearly with ADT for three zeolites. For every 20 ℃ increment of ADT, changes of DMM conversion were 10.4%, 15.6% and 17.4% for Y-15, Y-30 and Y-40, respectively. At the same ADT before reaching conversion plateaus, DMM conversion decreased significantly in the order of Y-40 > Y-30 > Y-15. This order suggested that the higher the Brønsted acid density (Table 1), the more difficult it was to recover the Brønsted acids at active sites from NH4+. The lowest density of Brønsted acids of Y-40 could also contribute to the slightly lower DMM conversion compared to Y-15 and Y-30 when the conversion plateaus were reached.

9



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Fig. 1. Influence of ammonia desorption temperature on (a) DMM conversion, (b) MMAc selectivity, (c) DME selectivity and (d) MF selectivity at 120℃ and 1.0 MPa CO.

Table 1 Structural Characteristics of Catalysts. Al

SA

wt%

(m2 g-1)

Y-15

2.35

Y-15-C

AlEF

Lewis acids

Brønsted acids

wt%c

(µ mol g-1)d

(µ mol g-1)d

0.83

7.4

93

236

-e

0.75

23

106

234

800

-

0.47

0

64

185

1.45

900

0.24

0.65

15.3

44

185

1.07

860

0.21

0.48

9.6

28

135

Ra

NH3/Al b

889

0.22

2.35

850

Y-15-CE

1.75

Y-30 Y-40

Cat

a

Ratio of the amount of ammonia desorbed above the threshold temperature

to the total desorbed ammonia. b

Ratio of the amount of total desorbed ammonia to the amount of total Al.

c

Calculated from 27Al MAS NMR spectra at δ=0 ppm.

d

Calculated from pyridine FTIR according to the reference.19

e

Not performed. Fig. 1b-1d show the selectivity of the products. MMAc selectivity on

zeolites with different Si/Al ratio increased quickly with ADT below the 10


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selectivity plateau temperatures. For example, the MMAc selectivity of Y-15 increased from 25.9% at 230℃ to 65.6% at 300℃ (the starting temperature of the selectivity plateau). In contrast to the MMAc selectivity, the MF selectivity decreased below the plateau temperatures. Within the same ADT interval of 230℃ to 300℃, the MF selectivity of Y-15 decreased from 30.7% to 8%. An experiment was performed to show that MF did not decompose during the reaction. The DME selectivity of Y-30 and Y-40 remained almost unchanged despite the large ADT changes, with only a slight increase at 500℃. The DME selectivity of Y-15 decreased in the initial period from 36% at 230℃ to 27.2% at 260℃ and showed little change at higher temperatures.

3.2. Zeolite treatment Extra-framework Al (AlEF) was present in Y-15, Y-30 and Y-40 (Table 1). Fig. 2 shows the

27

Al MAS NMR of Y-15, Y-15-C and Y-15-CE. Calcination of

Y-15 in static air was used for further dealumination.20,21 As shown in Table 1, AlEF increased from 7.4% in Y-15 to 23% in Y-15-C and the Lewis acid density increased correspondingly by 14%. It was noted that the Brønsted acid density of Y-15 and Y-15-C were almost the same after dealumination. After Na2EDTA treatment, the Brønsted acid density of Y-15 decreased by 21% compared to that of Y-15-CE. In addition, the Lewis acid density decreased significantly by 40% with the removal of 23% of AlEF. NH3/Al decreased by 37% after Na2EDTA treatment. When tested in DMM carbonylation, the Y-15-C and Y-15-CE product selectivities were similar to that shown in Fig. 1b-1d. The DMM conversion as a function of ADT is shown in Fig. 3. After dealumination, the DMM conversion curve shifted toward low ADT from Y-15 to Y-15-C before reaching the plateaus, just as the DMM conversion curve of Y-15 shifted toward that of Y-30 (Fig. 1a). 11



Fig. 2. 27Al MAS NMR spectra of catalysts.

Fig. 3. Influence of zeolite treatment on DMM conversion at 120℃ and 1.0 MPa CO.

3.3. DRIFTS results Y-40 was not studied in DRIFTS because the framework Brønsted acid density was too low to show spectral changes. Fig. 4a-4d show the DRIFTS of Y-15 and Y-30. All of the DRIFTS were difference spectra using ammonia-free 12


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zeolite as a background and were collected at 100℃ after NH3 desorption at different temperatures. When the band area decreased, NH3 desorbed from NH4+ and the amount of exposed Brønsted acids increased. A high frequency band at 3627 cm-1 and two low frequency bands at 3563 and 3552 cm-1, which are characteristic of Y zeolite,22 were found for both Y-15 and Y-30. A broad shoulder band at approximately 3533 cm-1 was obvious for Y-15. It was interesting to find that the band area of high and low frequency increased at low ADTs (Fig. 4a, 4c), especially for Y-15. This indicated that more Brønsted acids were covered by NH3 when ADT increased during Ar purging. The newly adsorbed NH3 could desorb from non-Brønsted acid sites and could be re-absorbed by free Brønsted acids at higher ADT. At high ADTs (Fig. 4b, 4d), the band area began to decrease gradually until it approached zero at 500℃, suggesting the complete recovery of Brønsted acids.

13



Fig. 4. DRIFTS of Y-15 at low (a) and high (b) ammonia desorption temperatures; DRIFTS of Y-30 at low (c) and high (d) ammonia desorption temperatures.

The spectra were analyzed by deconvolution. Four bands at 3627, 3622, 3563, 3552 cm-1 and one band at approximately 3533 cm-1 (3525-3546 cm-1) were obtained as individual components. Given the assignment of H-Y Brønsted acids by Miki Niwa 23 and the fact that bands except 3533 cm-1 were always noticeable and remained at the same frequency, bands at 3627 and 3622 cm-1 were assigned to Brønsted acids in a supercage and bands at 3563 and 3552 cm-1 were assigned to Brønsted acids in a sodalite cage and a hexagonal prism, respectively. The broad shoulder band at approximately 3533 cm-1 was assigned to Brønsted acids that were disturbed by AlEF species.24-26 The broad frequency region (3525-3546 cm-1) also contributed to this assignment.

3.4. NH3-TPD results DMM carbonylation occurred when the zeolite ADT was higher than the threshold temperature. To determine the amount of NH3 that was desorbed above the threshold temperature, NH3-TPD was performed with a constant temperature desorption stage at the threshold temperature.27-29 NH3-TPD results are shown in Fig. 5.

14


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Fig. 5. (a) Conventional NH3-TPD of zeolites with a heating rate of 10℃/min. (b) NH3-TPD as a function of desorption time. Two spots on each curve denoted 30-minute constant temperature stages that were 230℃, 200℃ and 190℃ for Y-15, Y-30 and Y-40, respectively. Before and after the constant temperature stages, the desorption temperature increased linearly with a heating rate of 10℃/min. For the same zeolite, the ammonia desorption amounts shown in Fig. 5a and Fig. 5b were the same. When the Si/Al ratio increased, zeolite NH3-TPD peaks of high and low temperatures shifted toward low temperatures. The left hand peaks in Fig. 5b occurred before the finish of constant temperature stages and indicated that NH3 was desorbed below the threshold temperatures. DMM carbonylation did not occur when ADT was below the threshold temperatures. DRIFTS also suggested that the framework Brønsted acids in the supercage, sodalite cage and hexagonal prism were not exposed until the ADT was above the threshold temperatures. Therefore, the left hand peaks correspond to NH3 that was desorbed from sites that were not framework Brønsted acids. These parts of NH3 could be adsorbed on ammonium ions that cover the framework Brønsted acids,30-32 framework Lewis acids

17,18

(as

suggested in Table 1 with a large amount, even for Y-15-CE), and extra-framework Lewis acids in the supercage and sodalite cage.20,33-35 The 15



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right hand peaks in Fig. 5b occurred after the culmination of the constant temperature stages and indicated that NH3 was desorbed mostly from Brønsted acid sites above the threshold temperatures. The Brønsted acids that correspond to these amounts of NH3 were total framework Brønsted acids in zeolites, including the supercage Brønsted acids that contributed to the reaction. Both of these amounts of NH3 and the desorption temperatures decreased in the order of Y-15 > Y-30 > Y-40, which was in accordance with the order of DMM conversion at the same ADT before the plateaus that are shown in Fig. 1a. It is interesting to note in Table 1 that the proportion of these amounts of NH3 to the total amounts of NH3 remained nearly the same (21-24%) for Y-15, Y-30 and Y-40, despite the large difference of NH3 desorption amounts and value of NH3/Al. If NH3 interacted with Brønsted acids with a ratio of 1:1, the amounts of total framework Brønsted acids in zeolites were the same as the amounts of NH3 that correspond to the right hand peaks in Fig. 5b, namely, 21%-24% of the amounts of total desorbed NH3. Given the amounts of Al and the value of NH3/Al that are shown in Table 1, the amounts of total desorbed NH3 could be calculated, and the amounts of total framework Brønsted acids were obtained as 10.1-18.3% of the total amount of Al. According to a study by Miki Niwa and coworkers,23 approximately 38.7% of total framework Brønsted acids are in a supercage, so the amounts of supercage

Brønsted

acids

that

contributed

to

DMM

carbonylation

corresponded to 7.1%, 6.0% and 3.9% of the total amounts of Al for Y-15, Y-30 and Y-40, respectively. If Brønsted acids that correspond to 3.9%-7.1% of the total amount of Al were assumed to be active sites, the maximum rates of MMAc synthesis at plateau temperatures were calculated as 131, 252 and 482 mol (mol active sites)-1 h-1 for Y-15, Y-30 and Y-40, respectively.

4. Discussion 16


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4.1 Effect of remaining NH4+ According to Alexis T. Bell and coworkers, the MMAc synthesis reaction involves carbocationic transition states that are stabilized by oxygen atoms in the zeolite framework. These carbocations might be influenced by the remaining NH4+ through electrostatic interactions,36-38 resulting in a decreasing stability of the carbocations and the dependence of MMAc selectivity on ADT. This influence should have existed until most NH4+ was decomposed. However, when the MMAc selectivity reached plateaus, DMM conversions were only 52%, 58% and 34% of the maximum conversion for Y-15, Y-30 and Y-40, respectively, suggesting that a significant amount of NH4+ was still present. DRIFTS results that are shown in Fig. 4b and 4d also showed that when MMAc selectivity plateaus were reached, a large proportion of the Brønsted acids were still not exposed from NH4+, suggesting again that the remaining NH4+ could have little effect on the stability of carbocationic transition states. Therefore, DMM conversion increased because more Brønsted acids were recovered from corresponding ammonium when ADT increased. Brønsted acids exposed above the threshold temperature contributed to all of the conversion. At higher ADTs, stronger Brønsted acids were recovered. The effect of ADT on the DMM conversion was the result of the gradual recovery of Brønsted acids, which indicated the Brønsted acid strength distribution. The dependence of MMAc selectivity on ADT was tentatively suggested as the difference of intrinsic properties of the exposed Brønsted acids. As shown in Fig. 1b, Brønsted acids exposed at low ADT catalyzed the reaction less selectively toward MMAc than those exposed at high ADT. As such, MMAc synthesis could require Brønsted acids of relatively high strength. The dependence of MF selectivity on ADT could result from the competition of MMAc and MF synthesis, both of which have methoxymethoxy species as a 17



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common intermediate.13 It was noted that the synthesis rate ratio of DME to MF was 4-5 at plateau temperatures, which was much larger than the DMM disproportionation value of two.12 This further demonstrated that different Brønsted acids among the same zeolite could have different activities. At the same ADT below the selectivity plateau temperatures, MMAc selectivity obviously decreased in the order of Y-40 > Y-30 > Y-15, which was the same trend as DMM conversion. Brønsted acids of Y zeolites with a lower acid density were recovered more easily from ammonium, and so at the same ADT, the stronger Brønsted acids were recovered more easily, resulting in higher MMAc selectivity. Therefore, the effect of Brønsted acid density on MMAc selectivity was likely to be the same as that of ADT.

4.2 Effect of AlEF Recent

studies

regarding

the

dealumination

mechanism

have

demonstrated that dealumination is a consecutive process.21,39 Four Al-O bonds bound to one framework Al can be gradually broken, and each broken Al-O bond is substituted by one Al-OH and one Si-OH band, with both being bound to the framework. The ultimate product is extra-framework Al(OH)3. Therefore, intermediate framework species with more than one Brønsted acid sites per Al site, such as Al-(OH)2, could exist. Brønsted acids decrease due to the framework Al decrease (15.6%) was compensated by Brønsted acid increase (21%) from the intermediate framework species. This was the reason that the Brønsted acid density of Y-15 did not decrease after dealumination. It has also been well studied that under mild conditions, such as those used in this work, part of the dealumination process proceeds reversibly.39-41 After Na2EDTA treatment, acid sites increase due to the dealumination process, 17% of Lewis acids (40%-23%) and 21% of Brønsted acids, disappeared. This reversibility was in accordance with a literature study as a result of sodium 18


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cation influence.42 The large decrease of NH3/Al was also attributed to the reversibility. Moreover, it suggested that the reversibility occurred because of the intermediate framework species. The results also indicated the different framework Al structures of Y zeolite after dealumination. After Na2EDTA treatment, the DMM conversion curve of Y-15-CE was very similar to that of Y-30 (Fig. 3). Given the results in Table 1 which show that Y-15-CE and Y-30 had the same Brønsted acid density and very different AlEF contents, it is suggested that there is a clear dependence of DMM conversion on the Brønsted acid density and that AlEF could have little effect on the reaction in the present study. Thus, synergy between AlEF and Brønsted acids was not noticeable in the present work. The DMM conversion of Y-15-CE was slightly higher than Y-15-C, which could be attributed to Brønsted acids of Y-15-CE that were more accessible after Na2EDTA treatment.

4.3 Turnover frequency of Brønsted acids The reaction results of the DMM conversion and product selectivity were obtained for zeolites HNH4-Y that were heated at different ADTs. The amount of Brønsted acids that were exposed and involved in the reaction depended on ADT. The higher the ADT, the more Brønsted acids were exposed to catalyze the reaction. Accurate quantification of the Brønsted acid amount at each ADT was difficult to achieve, so reaction rate data at each ADT were not obtained. Here, semi-quantitative results are discussed according to the devolution DRIFTS results. The DMM carbonylation was considered to proceed mostly in the supercage because Brønsted acids in supercage are much more accessible than those in the sodalite cage or the hexagonal prism.23,43,44 Considering the mild reaction conditions in the present work, the proton mobility of Brønsted acid sites between cages

45,46

could have little influence. All of the DRIFTS 19



shown were difference spectra with ammonia-free zeolite as a background, and were collected under the same conditions. The spectra showed changes in Brønsted acids that were gradually exposed with ADT. The area differences of the bands at the same wavenumber were assumed to semi-quantitatively indicate the changes in the Brønsted acid amount. Supercage Brønsted acids (bands at 3627 and 3622 cm-1) have similar extinction coefficients,23 so the amount was compared by the difference of the area sum of the two bands (Table 2). Table 2 Summary of the Devolution DRIFTS DMM

Temperature Areaa

Catalysts

conversion

(℃) (%)

Y-15

Y-30

a

230

3.34

3.6

300

2.66

40.3

360

1.41

67.9

240

1.19

24.5

280

1.04

63.2

320

0.56

76.7

Area sum of bands at 3627 and 3622 cm-1.

For Y-15, when ADT increased from 230℃ to 300℃, the area decreased by 20.4% compared to the total area decrease of 3.34 from 230℃ to 500℃. In addition, DMM conversion increased from 3.6% to 40.3%. It was reasonable to infer that 20.4% of the Brønsted acids contributed to 36.7% DMM conversion, nearly half of the maximum conversion of 76.9%. When ADT increased from 20


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300℃ to 360℃, however, 37.4% of the Brønsted acids contributed to the 27.6% conversion, with conversion per fraction of Brønsted acids decreasing to be 0.41 times as high as that from 230℃ to 300℃. Above the ADT of 360℃, the conversion per fraction of Brønsted acids was only 0.12 times as high as that from 230℃ to 300℃. Similarly, for Y-30, when ADT increased from 240℃ to 280℃, 12.6% of Brønsted acids contributed to the 38.7% conversion, nearly half of the maximum conversion of 79.4%. The conversion per fraction of Brønsted acids was 9.3 times as high as that from 280℃ to 320℃. Above the ADT of 320℃, the conversion increased slightly from 76.7% to 79.4% while the residual 47% of Brønsted acids were recovered. From the above discussion, it could be inferred that the conversion per fraction of Brønsted acids could have the same trend as TOF. In other words, TOF varied significantly among the Brønsted acids and decreased with ADT. The Brønsted acids that were exposed at lower ADT showed higher TOF. As discussed in section 4.1, the Brønsted acids exposed at low ADT catalyzed DMM carbonylation with the highest MF selectivity and the lowest MMAc selectivity. Therefore, this portion of the Brønsted acids catalyzed the side reaction most effectively, with both the highest selectivity and the highest TOF. Since Brønsted acid strength increased with ADT, it was suggested that the relatively weak Brønsted acids in high-silica H-Y catalyzed the side reaction more effectively, and the strong Brønsted acids catalyzed the main reaction more effectively. In addition, using the method of NH3 poisoning in this work, it was also possible to discriminate and calculate different acid sites in zeolite Y. It has been observed that only a small proportion of total Brønsted acids are active sites.36,37,47,48 However, the calculation of Brønsted acid amount is often roughly estimated by framework Al content and the kind of acid sites is not 21



distinguished quantitatively. The results of this work suggested that the amount of total framework Brønsted acids corresponded to 10.1%-18.3% of the total amount of Al. In addition, all of the supercage Brønsted acids were active sites. This finding was supported by reaction results, DRIFTS and NH3-TPD, all of which were performed with the similar catalyst pretreatment of NH3 desorption at the same temperature and a long enough desorption time of 30 minutes. According to the interaction with NH3, three kinds of zeolite acid sites existed in this work. The first kind was acid sites that did not interact with NH3. As shown in Table 1, the value of NH3/Al decreased with the ratio of Si/Al and was less than one, especially for Y-30 and Y-40. During the preparation process of high-silica Y zeolite, a significant amount of AlEF was trapped in cages, the influence of which was indicated by the disturbed low frequency at approximately 3533 cm-1 (Fig. 4). Giovanni Agostini et al. 34 demonstrated that 30-35% of the total Al are inside the sodalite cage after NH4-Y dealumination. These AlEF species acted as counter-ions, reducing the amount of protons associated with framework Brønsted acid sites. These AlEF and the corresponding non-acid framework Al sites likely did not interact with NH3. The other two kinds of acid sites were indicated by the left-hand and right-hand peaks in Fig. 5b, as shown in section 3.4. When it was assumed that NH3 interacted with acid sites at a ratio of 1:1, acid sites of different structures could be obtained quantitatively. Using Y-30 as an example, 35% of the total amount of Al that was inferred from the value of NH3/Al existed as the first kind of acid sites, and 49.4% and 15.6% of the total amount of Al existed as the other two types of acid sites.

5. Conclusions DMM carbonylation was catalyzed by HNH4-Y that gradually exposed Brønsted acids with ADT. The remaining NH4+ had little influence on the 22


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stability of carbocationic transition states. When the zeolite ADT was higher than 230℃, 200℃ and 190℃ for Y-15, Y-30 and Y-40, respectively, DMM carbonylation occurred. DMM conversion increased linearly with ADT as a result of Brønsted acid recovery before the plateau stage, which indicated the Brønsted acid strength distribution. MMAc and MF selectivity showed strong dependences on the Brønsted acid strength. Strong Brønsted acids catalyzed the main reaction more selectively toward MMAc, and weak Brønsted acids catalyzed the side reaction more selectively toward MF. Moreover, the TOF of Brønsted acids varied significantly among the Brønsted acids and decreased with acid strength. Weak Brønsted acids exposed at lower ADT showed higher TOF. Using this convenient NH3 poisoning method, it was shown that only 10.1%-18.3% of the total Al in high-silica zeolites Y contributed to the total framework Brønsted acids. These findings contribute to the understanding of high-silica H-Y catalyzed DMM carbonylation.

Acknowledgments This work was supported by the Projects of the State Key Laboratory of Coal Conversion of China (2013BWZ003) and the Project of Natural Science Foundation of China (NO.21303241).

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Study of the Nature of High-Silica HY Acid Sites in Dimethoxymethane

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